Most modern concretes are made with hydraulic cements generally obtained from Portland cement clinkers.
Portland cement is produced by heating a fine, intimate mixture of limestone, clay, silica and iron ore, to a temperature of over 1400° C. in a rotary oven. The calcined mixture, the clinker, takes the form of hard nodules which, after cooling, are ground with calcium sulphates and other added minerals to form the Portland cement.
The mixture of raw materials put into the oven needs to be very rich in limestone in order to obtain a clinker for which the main mineral phase is alite.
Alite is an impure form of calcium trisilicate, Ca3SiO5, for which the conventional notation is C3S.
A high percentage of alite, generally over 50%, is indispensable in the mineralogical composition of modern cements, because this is what allows the strength properties to develop rapidly just after setting, and allows the strength properties at 28 days and over to develop sufficiently, in order to meet the specifications, in this area, of most cement standards.
For the remaining of the description of the invention, the following abbreviated notations will be used, unless explicitly stated otherwise, to designate the mineral components of the cement.                C represents CaO,        A represents Al2O3,        F represents Fe2O3,        S represents SiO2,        $ represents SO3.        
Over the last decades, the level of carbon dioxide, CO2, in the atmosphere has increased considerably and continues to grow increasingly rapidly. This is linked to human activity, and scientists are unanimous in recognizing that this increase will have important effects on climatic conditions in the future.
Many governments today are taking steps to reverse the trend and are studying how to reduce CO2 emissions, particularly industrial emissions. The cement industry contributes greatly to these emissions, being responsible for 5% of all industrial emissions of CO2.
CO2 emissions in Portland cement clinker production can be reduced by about 10% if the alite is almost totally eliminated. This can be done if the quantity of limestone introduced into the oven is reduced by 10%; the quantity of CO2 linked to the decarbonatation of limestone during calcination is reduced, as is the amount of fuel necessary for supplying the energy to decarbonate the limestone.
This is accompanied by a reduced oven temperature, which has advantages, as described by E. Gartner, Cement and Concrete Research, “Industrially interesting approaches to low CO2 cements”, 2004, article in press CEMCON-02838.
Portland cement clinkers with a low alite content are always rich in belite, an impure form of calcium disilicate, Ca2SiO4, for which the conventional notation is C2S. But the belite-rich Portland cements obtained do not make it possible to obtain sufficient mechanical strength properties in the short term to meet standard requirements, nor to obtain the performance required at present from modern concrete applications.
For these reasons the production of belite-rich Portland cement clinkers are not a satisfactory solution for reducing industrial CO2 emissions by 10% or less.
In order to develop commercially useable cements, the production of which is associated with low industrial emissions of CO2, it is necessary to examine other types of hydraulic cement clinkers among these, systems based on calcium aluminates and/or calcium sulphates.
Alumina-rich cements, such as “Fondu Cement” by LAFARGE, are known for their property of acquiring high resistance in the short term; but they sometimes present the well-known problem of “conversion”, which is accompanied by a drop in the mechanical strength properties, and moreover highly specialised equipment is needed for their production, and a high fuel consumption, in spite of the low limestone content in the raw materials, and relatively expensive raw materials such as bauxite.
Besides, sulphate-based cements, such as gypsum and anhydrites, are inexpensive and generate little CO2 during their production, but cannot be used in most concrete applications, due to their low mechanical strength properties and their poor resistance to water.
However, certain types of cements based on calcium sulphoaluminates, written as CSA, are very important because they have simultaneously the positive effects of calcium aluminates and of calcium sulphates in terms of low industrial CO2 emission without having to use expensive raw materials, to the extent that the use of high quality bauxites could be minimised or be substituted by other materials.
Over the last 30 years, the Chinese cement industry has developed technology and set up a series of national standards concerning sulphoaluminous cements known as the “TCS series”, described by Zang L., Su M. Z., and WONG Y. M., in the journal “Advances in Cement Research”, Volume 11, no 1, 1999.
However, these cements have not been developed with the intention of reducing industrial emissions of CO2; they have mainly been developed for application in which high strength had to be obtained in the short term, as for prefabrication.
These “TCS series” sulphoaluminate cements are very rich in the calcium sulphoaluminate C4A3$ phase, known as “Klein salt” or “yee' limit”, which makes it possible to obtain high resistance in the short term, but in order to be formed during production, they necessitate introducing into the oven large quantities of high quality bauxite as a raw material. The cost of these cements is prohibitive for them to be used in many applications. Nevertheless, they can be produced with conventional rotary ovens.
The typical formulations of CSA aluminate cements are given in Table 1 below.
PhasesC4A3$ (%)C2S (%)C4AF (%)CSA (low ferrite55 to 7515 to 303 to 6content)CSA (high ferrite35 to 5515 to 3515 to 30content)                CSA: Sulphoaluminous cement.        
At the same time, P. K. Mehta in the USA developed other clinkers, the composition of which is based on the calcium sulphoaluminate phase C4A3$ “yee' limit”, and described in the journal “World Cement Technology” of May 1980, pp 166-177, and the journal “World Cement Technology” of July/August 1978, pp 144-160.
The clinkers described by Mehta differ from the “TCS series” mainly by their very high free calcium sulphate content in the form of anhydrite.
Although the clinkers described by Mehta have never been marketed, the clinker #5 reference quoted seems to correspond to the requirements of low industrial emission of CO2 and have performances that are roughly those of modern Portland cements.
This clinker contains 20% of C4A3$ “yee' limit”, 20% anhydrite C$, 45% belite C2S and 15% tetracalcium aluminoferrite C4AF.
However, in spite of the good performances obtained in the laboratory, this clinker and the others quoted by Mehta in his publications, have the disadvantage linked to their high calcium sulphate content; indeed, it is well known that calcium sulphate is unstable at high temperatures at which it dissociates, generating a gas, sulphur dioxide SO2, particularly in a reducing atmosphere or when the oxygen pressure is low, as is the case in rotary ovens. Therefore the clinkers proposed by Mehta would be difficult to produce in conventional rotary ovens without creating serious environmental problems related to the emission of sulphur dioxide SO2.
The clinker #5 quoted by Mehta in the journal “World Cement Technology” of May 1980, pp 166-177 has the following mineralogical composition, by weight compared with the total weight of clinker:                C2S: 45% C4A3$: 20% C4AF: 15% C$: 20%        with C$: calcium sulphate (anhydrite).        